Nebraska Revised Statute 72-304

The owner of the lease shall be required, in the case of potash, salines, silica, volcanic ash, sand, gravel, clay, fuller's earth, and tripoli, to report within thirty days after the issuance of such lease, and to begin construction of necessary works or the installation of necessary machinery, for the physical development of the area, within ninety days after the issuance of a lease. In the case of minerals, substances not above enumerated, and coal, the lessee shall report within six months after issuance of the lease, and begin development operations within one year from the date of the lease. The lessee shall make reports to the Board of Educational Lands and Funds showing the progress of the work on or before December 1 of each year.